A disconnectable mooring system of this type is disclosed in US2007155259. This system includes a buoy member that is provided with a conical outer casing and a receptacle of the turret structure has a cone shape corresponding to the conical outer casing of the buoy member. The turret structure includes a turntable carrying conduits to be connected to the risers, wherein the turntable is supported on a bearing assembly in a manner allowing rotation with respect to the turret structure to align the conduits with the risers only after the buoy member is received and locked in the receptacle of the turret structure. In this patent publication it is shown that the turntable is supported by a main turret upper roller ball bearing assembly only. This bearing assembly includes three mutually movable parts that are directly interconnected to each other. In fact, this upper turret bearing assembly consists of 2 roller ball bearings that are directly placed on top of each other and interconnected via one common inner bearing housing member. This known upper bearing assembly has therefore become a critical and essential part of the turret-moored weathervaning system. A disadvantage of integrating the turret bearing and the manifold bearing into a single bearing structure is that if one or more roller balls fails, the complete bearing assembly has to be replaced, meaning that the turret system can no longer function as a weathervaning system. This replacement cannot be carried out under offshore conditions, so that the vessel needs to be transported to a on shore location for repair.
Another disadvantage of the known system is that the very heavy turntable with several manifold and swivels decks is at all times supported by this very sensitive rolling bearing assembly. Consequently all the large static and dynamic forces both in a radial and in an axial direction, and moments resulting form the turntable deck structure and the environment, are directly transferred to this critical roller ball bearing system, which is known in the industry to be very sensitive to wear and fatigue. Another disadvantage of this integrated roller bearing system is that the inner diameter due to the fabrication limitations, is limited to a maximum dimension of only about 8 meters, so that it not suitable for large disconnectable turret-buoy systems which comprise for example 20 or more risers connected to the buoy.
Another patent publication that describes a disconnectable mooring system that is provided with two separate bearing systems, one of which is used only for rotating a turntable in order to align the manifold pipe ends with the riser ends of a connected buoy, is U.S. Pat. No. 5,651,708. This patent shows a disconnectable buoy that is provided with a bearing system, which stays with the boy after it is disconnected. The buoy is rotatably connected to the moonpool of a vessel under the waterline without the use of a turret. An additional upper bearing system is disclosed at deck level, which supports a turntable with manifold, so that after the buoy is connected directly to the moonpool of the vessel, the turntable can be aligned with the risers of the connected buoy. The turntable is supported by the bearing system, so that even during production when hydrocarbons are received through the flexible piping connecting the manifold and the buoy, the turntable can be rotated at all times and be aligned with the buoy. When the twisting angle in the flexible piping between the buoy and the turntable is exceeded, the turntable is rotated by means of a connected motor driven pinion to a new position neutralizing the twisting. This system is therefore not suitable for disconnectable turret-buoy systems sized to receive larger numbers of risers, and cannot be used when using only hard piping to connect the risers and manifold.
It is therefore an object of the present invention to provide a rapidly disconnectable and easy connectable mooring buoy system that is able to support a large numbers of risers, for example at least 20 risers and 10 umbilicals. It is a further object of the present invention to provide a turret mooring system that is reliable in operation and provides a better distribution of forces on the bearing and hence reduces the risk of down time by maintenance activities. The mooring buoy system of the present invention should readily connect and disconnect even under severe environmental conditions to a floating vessel, for example a floating production unit (FPU or FPSO). The present invention can be applied for a disconnectable turret mooring system that will be permanently integrated towards the bow of a Floating Production Unit (FPU). The system allows the vessel to passively weathervane around the anchor legs and to take up the position of least resistance to the prevailing weather, while simultaneously transferring fluids, gas, power, and communications signals between the FPU and the subsea equipment. The system's design incorporates a disconnection/reconnection capability to allow the FPU vessel to disconnect from its anchoring system when for instance iceberg warnings are issued.